This invention relates to catalyst inventory measurements and, more particularly, to a process for determining the amount of catalyst in a resid hydrotreating unit in an oil refinery.
In the past, spiraling oil costs, extensive price fluctuations, and artificial output limitations by the cartel of oil producing countries (OPEC) have created instability and uncertainty for net oil consuming countries, such as the United States, to attain adequate supplies of high quality, low-sulfur, petroleum crude oil (sweet crude) from Saudi Arabia, Nigeria, Norway, and other countries at reasonable prices for conversion into gasoline, fuel oil, and petrochemical feedstocks. In an effort to stabilize the supply and availability of crude oil at reasonable prices, Amoco Oil Company has developed, constructed, and commercialized extensive, multimillion dollar refinery projects under the Second Crude Replacement Program (CRP II) to process poorer quality, high sulfur, petroleum crude oil (sour crude) and demetalate, desulfurize, and hydrocrack resid to produce high-value products, such as gasoline, distillates, catalytic cracker feed, metallurgical coke, and petrochemical feedstocks. The Crude Replacement Program is of great benefit to the oil-consuming nations by providing for the availability of adequate supplies of gasoline and other petroleum products at reasonable prices while protecting the downstream operations of refining companies.
The successful commercialization and use of ebullated bed reactors requires numerous tons of catalyst to be transported to and removed from the ebullated bed reactors daily. It is important to have accurate measurements of the catalyst inventory in the ebullated bed reactors since such measurements determine the amount of fresh catalyst to be added to the reactors and the amount of spent catalyst to be withdrawn from the reactors. It is also important to have an accurate measurement of the catalyst inventory to apply process models to the overall process to most efficiently operate the unit for maximum product yield and process economy. The amount of catalyst inventory is also essential to determine the space velocity, which is the ratio of oil flowrate to catalyst volume. Actual catalyst inventory determination with little or no error is required to accurately determine the amount of fresh catalyst replacement to most effectively and productively operate the reactors.
For example, if an extra 1% of fresh catalyst is added per day to the reactors, the reactors will be completely filled in a short period of time. If 1% less than the proper amount of catalyst is added per day to the reactors, the reactors will be completely empty of catalyst in 100 days, assuming an initial load of 100%.
Inadequate amounts of a catalyst can lead to excessive thermal cracking, increased catalyst coking (coke formation), excessive solids buildup, and plugging. This can lead to shutdown, extended downtime, increased frequency of repair, decreased efficiency, and reduced product quality.
Excessive catalyst inventory can produce packed beds and local hot spots. Packed bed reactors are also much more susceptible to plugging by any solids present. Too much catalyst occupies portions of the reactor volume needed to convert the oil feed to more valuable products. Excessive catalyst can also plug the pumps, lines, and reactor grids. Furthermore, excessive catalyst can lead to premature shutdown, extended downtime, and increased frequency of maintenance and repair. Increased maintenance and repair requires additional manpower and is time consuming, tedious, and expensive. Excessive catalyst inventory also diminishes the conversion of feed to more valuable lower-boiling liquid products. Excessive catalyst further decreases the reactors' efficiency and adversely affects the profitability of the unit.
One method which has been suggested to measure catalyst inventory is to insert a plumb line and bob (bobber) to measure the distance between the top of the catalyst bed and the top or upper tangent line of the reactor. The amount of catalyst can then be determined by calculating the empty space (volume) above the catalyst bed based upon the measured height and the known effective area of the reactor. The plumb and bob technique requires that the reactor be opened to insert the plumb and bob. Since the reactor is operated at high pressures in nonoxidative or inert conditions, this will cause the pressurized hydrocarbon contents of the reactor to flow out of the reactor to the surrounding air. This can rapidly lead to flammable limits and explosive conditions which can cause explosions, fire, and damage to the reactor and injury to surrounding personnel.
The plumb and bob technique can also be used after first depressurizing the reactor to minimize explosive conditions. Unfortunately, this leads to excessive downtime and decreased profitability of the unit. Depressurizing of the reactor will also cool the reactor as well as the oil feed which can cause dissolved hydrogen to degas from the metal walls and liners of the reactor which may cause substantial damage to the structural integrity of the liner.
It is, therefore, desirable to provide an improved process for determining the catalyst inventory in an ebullated bed reactor which overcomes most, if not all, of the preceding problems.